The slow-twitch muscle fibers got bigger no matter what kind of workout or drink the men used—whether they lifted slowly up or down, or drank protein or sugar water.

When young men did leg extensions with a slow upward motion and drank a protein shake right after, their fast-twitch muscle fibers got noticeably bigger—but this didn’t happen if they did the slow downward motion or drank sugar water instead.

Even though muscles get bigger after 10 weeks of weightlifting, the individual muscle fibers don’t necessarily get thicker — the growth might come from how the fibers are arranged, not from each one swelling up.

Even though muscles got bigger, the actual muscle fibers didn’t get thicker or change type—so the growth might be due to other factors like fluid or connective tissue.

Both light and heavy lifting can boost the number of muscle repair cells in slow-twitch fibers, which might help muscles recover or adapt over time.

If you lift heavy weights or do lots of reps with lighter weights, your muscles grow at the same rate—so it doesn’t matter which way you train, as long as you’re already fit.